Fill = Random and not actual fill

So my friends and I have played a normal draft game today where 2 of us had chosen fill and 1 mid, 1 adc and 1 top. The first fill got support, but I got MID, even though we are 5 premade and my friend had chosen mid specifically not mid and fill, just mid but he got jungle and I got mid which makes no sense because I was supposed to get jungle.

The second game 1 friend had chosen jungle and 4 of us had chosen fill. And guess what, the friend who had chosen jungle got adc and the others got the remaining roles including jungle which he specifically had chosen.

It wasn't much of a problem for us because we switched anyway because we were 5 premades but I know there are people who use fill in ranked to gain advantage so their teammates won't be autofilled. But it seems like this doesn't happen and riot is forcing autofill even when it's absolutely not needed. I'm not sure if this is a bug or intentional but it definitely shouldn't be like this.